British prime minister David Cameron says he is delighted by the "absolutely wonderful news".

"I'm sure they'll make absolutely brilliant parents and I'm sure everyone around the country will be celebrating with them tonight," he said.

Prime Minister Julia Gillard also sent her congratulations to the couple.

"This is delightful news," she said.

"It's going to bring joy to them and to their family and I think it's going to bring joy to many around the world.

"Clearly it's a time of joy, it can also be a time of challenge and I'm sure many will be thinking of Kate as she deals with morning sickness and is in hospital."

US president Barack Obama also sent his congratulations to the royal couple.

"On behalf of everyone here in the White House, beginning with the president and the first lady, we extend our congratulations to the Duke and Duchess of Cambridge on the welcome news this morning out of London that they are expecting their first child," White House spokesman Jay Carney said.

The Obamas met the royal couple, who were then newlyweds, during a state visit to Britain last year.

"I know they both feel that having a child is one of the most wonderful parts of their lives, so I'm sure that will be the same for the Duke and Duchess of Cambridge," Mr Carney said.

Heir to the throne

The royal baby will be born third in line to the throne, after Prince Charles and Prince William.

If the baby is a girl, she will be Britain's next queen, regardless of any brothers she might have.

Last year Britain and other Commonwealth countries which have the Queen as their monarch agreed to change the rules of royal succession so that males would no longer have precedence as heir, regardless of age.

The agreement also means an end to a ban on a future monarch marrying a Catholic, a stipulation dating back about 300 years.

He or she will one day be head of the British armed forces, supreme governor of the Church of England and head of state of 16 Commonwealth countries.

Author and royal watcher Phil Dampier told AM he expects the announcement will boost the monarchy's standing in Britain.

The arrival of a new baby is going to put the interest into the stratosphere, to be honest. It's going to be Diana-mania all over again.

I think it is going to be enormous and I have to say it's probably putting back the republican cause even further. I can't really speak for Australia obviously but I know in other countries the monarchy's had a fantastic boost and Down Under they seem to be more popular than ever before.

The press is very much in the dock over here at the moment and it's going to be very, very difficult for them to restrain themselves and be sensible and make sure that they don't cross the line and try and get pictures of Kate with a bump or try and go over the top and get pictures that she doesn't want taken.

I think the palace will probably have to be quite skilful at providing a lot of material for the press to give them something to write about, because otherwise it could leave open a bit of a vacuum for people to misbehave.

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